LIVERMORE -- Firefighters are working to contain a grass fire that has burned 23 acres near Interstate 580 at Grant Line Road near the Altamont Pass.

Cal Fire chief Jim Crawford said firefighters had contained about 95 percent of the fire that was reported at 12:01 p.m. Saturday afternoon. No structures were threatened, but officials briefly shut down a section of railroad nearby.

The cause is unknown and fire officials are investigating the blaze.

Check back for updates.